Yeast and beef heart cytochrome oxidases will be subfractionated into their component subunit proteins. Antisera prepared against the purified components will be used to study the functional and structural roles of the subunits by immunochemical methods. Special emphasis will be placed on further characterizing the heme peptide of the oxidase. A second aspect of the proposal deals with studies on the mechanism of assembly of cytochrome oxidase in yeast. A screening program will be devised to isolate temperature-sensitive mutants of Saccharomyces cerevisiae with specific lesions in cytochrome oxidase. Such mutants will be used to probe the in vivo sequence of assembly of the subunits into an active enzyme.